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[lvm-devel] Re: Deactivating volume with mounted snapshot



On Thu, 15 May 2008, Alasdair G Kergon wrote:

On Wed, May 14, 2008 at 07:30:17PM -0400, Mikulas Patocka wrote:
With merging, it will cause corruption --- if one snapshot is merging and
you don't have all the other snapshots loaded, the non-active snapshots
won't be updated as the merging goes on. So they'll be corrupted.

If that can happen it suggests the detailed set of activation steps needs
further refining and is not correct yet.

I think I have it already correct. The patch that corrects it is http://lacrosse.redhat.com/~mpatocka/patches/kernel/dm-snapshot-remove-active-variable.patch

Basically, lvm suspends all the devices, then loads the dm table for all of them and then resumes them.

I start merging at preresume method. When I start merging, all the other snapshots are already loaded in the table and can allocate new exceptions. The patch changes it so that they can accept new exceptions always.

Currently the contstraint is "if the device is open, you can't deactivate
it". I'd like change it to "if the device is open or some of its snapshots
are open, you can't deactivate it".

Catch-22 then.  Which do you deactivate first?  We cannot handle that
at kernel level with the current interface.

You are right. The possible solutions:

* block opening of suspended device until the device is resumed (or deleted) --- easiest, but most scary. Theoretically it should be OK --- you already should not open a file while there is something suspended (the memory-allocation deadlock) --- but it may make some of these deadlocks very visible. Maybe making the deadlocks visible is a good thing, it's much easier to solve reproducible deadlock than nonreproducible.

* undo the deactivation if one of the devices can't be removed because it is open --- probably the hardest.

* replace the open device's table with error target --- if it has journaled filesystem, it will recover fine. And it won't damage other snapshots.

* let it be, hoping that admin won't simultaneously open and deactivate a device :-/

But by all means add that extra protection to the userspace code.
The userspace code should not assume it happened though.

Even without merging, the current approach is not perfect, as you will end
up with a device that you can't deactivate (the origin is deactivated, the
snapshot is active; lvchange -an on the snapshot will return error;
lvchange -an on the origin will see that it is already inactive and do
nothing)

That can be fixed: lvchange -an on the origin should complete the process.
(It probably used to do that before some optimisation got added.)

Alasdair
--
agk redhat com

BTW. Tomorrow I'm going to Brno to medical exam at Red Hat on Thursday and meeting on Friday, so I won't be on IRC and email much.

Mikulas


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